Sulfur-graphene oxide material for lithium-sulfur battery cathodes

Sulfur-graphene oxide material for lithium-sulfur battery cathodes

June 06, 2013

Searching for a safer, less expensive alternative to today's lithium-ion batteries, scientists have turned to lithium-sulfur as a possible chemistry for next-generation batteries. Li/S batteries have several times the energy storage capacity of the best currently available rechargeable Li-ion battery, and sulfur is inexpensive and nontoxic. Current batteries using this chemistry, however, suffer from extremely short cycle life—they don't last through many charge-discharge cycles before they fail.

A research team led by Elton Cairns and Yuegang Zhang has developed a new material for the cathode (the positive electrode) that could lead to practical lithium sulfur batteries. Cairns is a researcher in the Environmental Energy Technologies Division (EETD), and Zhang, in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory (Berkeley Lab).

Lithium ion cells are reaching their maximum energy storage capability (~200 Watt-hours/kg) and are still not able to provide a safe, low-cost battery of sufficient energy storage capacity for electric vehicles of more than 100-mile range, and sufficient operating time for many mobile applications, including laptop computers, tablets, and cell phones.

To continue the progress toward electric vehicles, as well as toward higher-capacity batteries for mobile electronic applications, the marketplace needs a new generation of battery with a specific energy of at least 400 Watt-hours/kilogram (Wh/kg), low cost (under $200/kWh), improved safety, and low environmental impact.

The Li/S cell offers a very high theoretical specific energy (2,680 Wh/kg), much higher than that of the best Li-ion cell (~580 Wh/kg). (See Figure 1.) Thanks to this high energy per weight, Li/S batteries could store more energy, and therefore, provide greater vehicle range as well as longer operating times in all applications.

Sulfur is inexpensive, non-toxic, safe, and environmentally benign, so Li/S batteries would be cheaper than current Li-ion batteries, and they would be less prone to safety problems that have plagued some of today's Li-ion batteries, such as overheating and catching fire. But something needs to be done to overcome their current short cycle life—most prototype Li/S batteries lose their ability to store charge after a dozen charge-recharge cycles, or less.

Barriers to Li/S cell progress

The brief cycle life has been the main impediment to commercialization. The technology developed by Cairns, Zhang and colleagues to address this problem is a sulfur-graphene oxide nanocomposite material (S-GO) for use as the battery's cathode.

S-GO is a nanocomposite material in the form of small particles of sulfur-coated graphene flakes that is designed for use as the cathode in a lithium/sulfur battery. As the cathode material, S-GO binds with lithium during the battery's discharge cycle. During battery recharge, lithium returns to the battery's negative electrode.

Overcoming short cycle life and volume changes

The cause of Li/S batteries' short cycle lives is that sulfur is highly soluble in the organic solvents typically used as electrolytes, forming lithium polysulfides. The polysulfide ions can diffuse through the electrolyte to the lithium anode where they can form precipitates such as Li2S. This phenomenon reduces the utilization of active materials at the electrodes and shortens battery life.

Another barrier is the significant volume increase, 76 percent, which takes place as sulfur is converted to Li2S during discharge, causing loss of electronic contact of the sulfur with the current collector from mechanical degradation.

S-GO's unique structure improves its performance as a cathode material for Li/S batteries. It can accommodate the volume change of sulfur as it is converted to Li2S on discharge, and back to elemental sulfur on recharge, while maintaining the bonding between the graphene oxide surface groups and the sulfur.

The large surface area of S-GO, along with its ubiquitous cavities, establishes more intimate electronic contact with sulfur and avoids particle aggregation and loss of electrical contact with the current collector. (See scanning electron microscope image of S-GO, Figure 2.)

The graphene oxide contains functional groups (including epoxy and hydroxyl), which can strongly anchor sulfur atoms and effectively prevent the lithium polysulfides from dissolving the electrolyte during cycling, which would otherwise lead to the battery's rapid failure. Current tests of the technology have shown that Li/S batteries with S-GO cathodes had stable cycling for many hundreds of deep cycles with high sulfur utilization.

Potential benefits of Li/S batteries

Lithium/sulfur batteries could revolutionize both electric transportation and the electric grid. Because they can store four times more energy than current lithium ion batteries per unit of weight (600 to 800 Wh/kg), they could extend the range of an electric vehicle to that of a gasoline powered car—300 to 400 miles on a single charge. Sulfur is inexpensive, less than $1.00/kg, and Li/S batteries would be significantly lower in cost than Li ion batteries, creating the potential for rapid and high market penetration.

Stationary battery banks using Li/S technology could provide the high storage capacity at low cost needed to make large-scale stationary storage of electricity from wind and solar power possible on the grid.